Application of electron spin resonance to determination of serum drug concentrations.

Displacement of spin-labeled drugs from drug-specific antibodies in human serum can be measured by use of electron spin resonance. This "spin immunoassay" has advantages of speed, simplicity, and small volume requirements over other standard techniques such as gas-liquid, liquid-liquid, or thin-layer chromatography. Unlike radioimmunoassay, there is no need to separate free and bound label and there are no radiation hazards. We present clinical and experimental data showing how this method can be applied to the routine determination of morphine and diphenylhydantoin concentrations in serum.